Pin-hole evaporation camera



March 31, 1964 c. w. RECTOR PIN-HOLE EVAPORATION CAMERA Filed Oct. 4.1960 III I INVENTOR. (241x45: W kicrox BY United States Patent 3,127,226PIN-HOLE EVAPURATION CAMERA Charles W. Rector, Baltimore, Md., assignorto Radio Corporation of America, a corporation of Delaware Filed Get. 4,1960, Ser. No. 66,433 3 Claims. (Cl. 316-4) This invention relates to amethod of and means for forming thin deposits. The invention will bedescribed with particular reference to the deposition of thinphotoconductive deposits for which it is particularly useful.

In several known electronic devices, e.g., photoconductive pickup tubesand photocells, it is desirable to obtain a photoconductive deposithaving a maximum sensitivity combined with a minimum photoconductivelag, maximum speed of response, selected spectral response as well asother preferred parameters of the photoconductive deposit. One of thesolutions that has been proposed to obtain the photoconductive depositshaving these desired properties is the simultaneous use of two or moredifferent photoconductive materials. For example, it has been proposedto use a photoconductive material including antimony tri-sulfide andantimony oxy-sulfide. At times when using the plural material deposits,it is desirable to deposit the different photoconductive materials astwo layers one on top of the other. For other situations it is desirableto deposit the materials as a graded composition in which thecomposition of each incremental thickness element varies as a functionof the distance through the layer.

It is apparent that, without a knowledge of the relative rates at whichthe various components of the plural photoconductive materials are beingdeposited, control and reproducibility of a desired photoconductivememher would be exceedingly difiicult to attain. Since thephotoconductive materials are usually deposited by evaporation in sometype of vacuum, factors which are known to affect the rate of depositionof a material include the following: (1) the chemical composition of thematerial; (2) the evaporating temperature of the material; (3) thespacing between the evaporator and the surface on which the material isbeing deposited; (4) the degree of vacuum through which the evaporationis occurring; (5) the configuration of the evaporator; (6) the length oftime of the evaporation; (7) the temperature of the evaporant moleculesas they traverse the space between the evaporator and the surface onwhich the material is being deposited; and (8) the type of gas moleculesthrough which the evaporation is occurring. As is obvious from thelength of this incomplete list of factors aifecting the rates at whichvarious components are deposited, it is extremely difiicult to controlthe amount of material deposited and therefore the reproducibility of aphotoconductive member.

It is therefore an object of this invention to provide a new method ofand novel means for monitoring the amount of material deposited.

It is another object to provide a novel method of and means fordepositing a photoconductive member, in-

eluding a plurality of photoconductive materials.

It is a further object to provide a new and improved device having aphotoconductive member made by the novel process of this invention.

These and other objects are accomplished in accordance with thisinvention by providing a novel device which enables the relative amountsof each of the components of a multiple component co-evaporation to beseparately monitored While the various materials are being depositedupon a common selected surface. To achieve this, the device includes atleast two spaced evaporator sources and an aperture positioned in theevaporant stream from both of the evaporators. By spacing a monitoringplate behind the aperture, separate, clearly defined images of theamount of material being deposited on the selected surface, are alsodeposited on the monitoring plate. By monitoring each component duringthe evaporation process, such as by counting the interference bands asthey are formed on the monitoring plate, the amount of the differentmaterials deposited on the selected surface can be accuratelydetermined. With the separate amounts of the different depositedmaterials known, rates of evaporatron of each component can be adjusted,while. the evaporation is taking place, to obtain any desiredcomposition or graded composition through the thickness of thephotoconductor deposit.

The invention will be more clearly understood by reference to theaccompanying single sheet of drawings where- 1n;

FIG. 1 is a sectional view of a pickup or camera tube having a targetelectrode made in accordance with this 1nvention;

FIG. 2 is an enlarged fragmentary sectional view of the target electrodeshown in FIG. 1;

FIG. 3 is an enlarged fragmentary sectional view of another embodimentof a target electrode made in accordance with this invention;

FIG. 4 is a partially schematic sectional view of means for obtainingthe target electrodes shown in FIGS. 2 and 3 in accordance with thisinvention; and,

FIG. 5 is a broken away perspective view of an evaporator boat for usesin the means shown in FIG. 4.

Referring now to the drawings in detail, there is shown, in FIG. 1, aphotoconductive type pickup tube it). The pickup tube It is an exampleof a tube wherein this invention is particularly useful and theinvention will be explained in detail in connection with this type oftube. The tube lil comprises an evacuated envelope 12 having an electrongun assembly 14 positioned in one end thereof for producing an electronbeam. By means of potentials applied to the electron gun l4- and to anaccelerating electrode 16, as well by means of appropriate magneticfields from conventional alignment coils, focus coils and a deflectionyoke, none of which are shown for simplicity of illustration, theelectron beam is directed toward and scanned over a photoconductivetarget electrode lid.

The target electrode 18, which is shown more clearly in FIG. 2,comprises a transparent electrically conductive electrode 20 depositedon a transparent face plate 22. The face plate 22 forms an end of theenvelope 12. The transparent electrically conductive electrode 20 ismade of a material that is selected for its transparency to radiationsof the particular wavelengths of interest and for its electricalconductivity. For the visible range of wavelengths, a layer of tin oxidehas been found to be suitable. The transparent conductive coating 29 isdeposited in contact with an electrically conductive sealing ring 24 sothat, during operation of the device 10, an electrical potential may beapplied to the transparent conductor 2t and the transparent conductormay function as a signal plate for obtaining output signals from thedevice 10.

On the transparent conductor 2d there is deposited a graded compositionof photoconductivee material 26. In this particular embodiment, thegraded composition of photoconductive material comprises a layer 26a offirst photoconductive material, a layer 26b of a mixture of the firstand of a second photoconductive material, and a layer 26c of the secondphotoconductive material. The particular chemical compositions selectedfor the various layers of the graded composition of photoconductivematerial 26 may be any known chemicals. One example of a gradedcomposition photoconductor is antimony oxysulfide used as the layer 26a,antimony tri-sulfide used as the layer 260, and a mixture of the two forthe intermes13 diate layer 26b. Other known photoconductive materials,or mixtures of materials, may also be used since, by means of thisinvention, any selected thickness of a layer, or mixture of layers, maybe accuratelycontrolled. Thus, by means of this invention, aphotoconductive target electrode 18 may be constructed with the mostdesirable optical and electrical properties.

Referring now to FIG. 3 there is shown an embodiment of aphotoconductive target including two separate distinct layers 28 and 30of different photoconductive materials. The layer 28 of aphotoconductive material may be any selected chemical compound ormixture of compounds and may itself be a graded composition. The layer30' of photoconductive material may also be any selected chemicalcompound or mixture of compounds and may be the same as or diflerentfrom the layer 23. Examples of photoconductive materials which may beused in accordance with this invention and which have been deposited intubes of the type shown in FIG. 1 include the oxides, sulfides, andselenides of antimony, lead, arsenic and cadmium, as well as mixtures ofthese materials.

Thus, targets of any number of layers of materials, with each layerbeing made of one or more chemical compositions, and graded in anydesired manner, may be manufactured by using this invention.

An example of a particular means for accurately forming the selectedphotoconductive layers in accordance with this invention is shown inFIG. 4. In this example two separate face plates 22, 22:: are in theprocess of having a photoconductive target electrode deposited thereon.It should be understood that the photoconductive materials can besimultaneously deposited onto any number of face plates and two areshown for simplicity of illustration. Prior to the manufacturing stepshown in FIG. 4-, a transparent electrical conductive coating 20 hasbeen deposited on each of the face plates 22, 22a, by any known means.

To deposit the photoconductive target electrodes 18,

the face plates 22, 22a, with their transparent conductive coatings 20,are positioned on a jig 28 and in an evacuated chamber (not shown). Alsopositioned in the evacuated chamber are at least two spaced apartevaporator boats 30. The structure of the evaporator boats 30 willsubsequently be explained in detail in connection with FIG. 5. It shouldbe noted that the jib 2.8 which supports the face plates 22, 22a,includes a small, e.g. approximately 0.07 inch aperture 32. Positionedin spaced relation above the aperture 32 is a monitoring plate '34. Thejig 28 may be made of a material such as stainless steel while themonitoring plate 34 may be made of any transparent material, onesuccessfully used example of which .is an optically flat transparentglass.

The vacuum chamber is evacuated to a relatively high vacuum, e.g. 10*mm. of Hg, for some target materials. At this time, current is passedthrough the evaporator boats 30 to heat these boats to a temperaturehigh enough to evaporate the material contained therein. In the highvacutun, the evaporation process involves essentially a rectilinearpropagation of the evaporant. Thus, those evaporant molecules comingfrom an evaporator boat 30 which pass through the aperture 32 land onlyon a small region or area of the monitoring plate 34, the extent ofwhich is determined primarily by the dimensions of the object source,the size of the aperture and the distance.

The image formed on the monitoring plate 34 is, in fact, a geometricalrepresentation of the relative rates of evaporation from variousportions of the evaporating area. A long evaporation boat, for example,will give a long image; while any hot spots in the evaporation boat willshow up as heavier deposits in the corresponding parts of the image. Inan example of an evaporation unit used for the evaporation of anyantimony tri-sulfide antimony oxy-sulfide target, a spacing betweenboats 30 of 3.3 cm. was used with the boats arranged concentricallyaround the axis of the aperture 32. With the boats 30 spacedapproximately 185 cm. below the aperture and the monitorll ing plate 34positioned approximately 3.5 cm. above the aperture, the spacing betweenthe deposits of material on the monitoring plate was approximately 6.25mm.

The amount of material deposited on the monitoring plate 34 isproportional to the amount deposited on the tar-get 18. The amount ofmaterial comprising each image on the monitoring plate is mostconveniently monitored optically. Thus, a light source 36 directs lightonto both of the photoconductor images and the reflected, ortransmitted, light from each is monitored either by an observer orelectrically.

With transmission monitoring, highly absorbed green or blue light fromthe source 36 should be used for the important first stages of theevaporation. In the later stages, when an appreciable amount ofphotoconductive material has been deposited on the monitoring plate, redor near infra red light should be used.

A light source which has been used as the source 36 .is a whitefluorescence bulb such as used for ceiling light- .ing fixtures. Thisoffers a fairly continuous light spectrum and, at the distance used,forms a broad area source illuminating the whole of each image on themonitoring plate. The reflection from the image on the monitoring plateshows interference bands, of the deposited material. The interferencebands form at the center of the image and expand as separate rings, asthe evaporation proceeds. These interference bands, since a white lightis used, are subtraction colors and occur in the sequence yellow, red,blue. The sequence then proceeds as lY, 1R, 1B, 2Y, 2R, 2B, 3Y, 3R, 3Buntil the termination of the evaporation. With careful observationtechniques an 83 hand can be detected by the human eye when the materialis antimony tri-sulfide. Since the antimony oxy-sulfide is lessabsorbing, an even greater number of bands can be detected, whenusing'this material. An examination of the monitoring plate with a handlens after removal from the vacuum system establishes any doubtful finalbands.

A specific example of a target for thexvisible range of the spectrum,made in accordance with this invention, and using available materials,is as follows: The first deposit (deposit 28 shown in FIG. 3) was madeof antimony oxy-sulfide and was deposited until the fourth orderintereference ring was formed. Then, antimony tri-sulfide and antimonyoxy-sulfide (corresponding to deposit 30 in FIG. 3) were simultaneouslydeposited,

both at a substantially uniform rate, until the fifth order interferencering of antimony oxy-sulfide and the seventh order interference ring ofthe antimony tri-sulfide were reached. It should be understood that whenmaterials are used, other than those referred to above, a particularnumber of interference bands will not necessarily indicate the samethickness of photoconductive deposit. However, with the index ofrefraction of the selected material known, and the number ofinterference bands known, the amount of material deposited may beapproximately obtained by the following relationship:

N is the number of counted interference bands.

11 is the index of refraction of thephotoconductive material (3.5 forantimony tri-sulfide and antimony oxysulfide).

7\ is the light wavelength in air (0.55 micron).

L is the distance from the evaporator boat to the face p ate.

L is the distance from the evaporator boat to the monitoring plate.

Thus, in the above example with L 21.5 -1.4a

and the thickness in microns=.ll2N. The result of this \43 is that, withthe fourth order interference band of antimony oXy-sulfide, thethickness was approximately .45 micron; and the second deposit thicknessof a mixture of the two materials (N=+7) was 1.34 microns. It should beunderstood that this invention is not limited to the above specificexample. The total thickness of both layers was 1.79 microns.

The above formula is correct only if the emission area of the evaporatorboat is small enough so that the center of the monitor spot receivesmaterial from the entire emission area of the evaporator boat. Thiscondition is fulfilled by the evaporator boat of FIG. 5 and the otherapparatus used in accordance with this invention.

When the evaporation is complete, the face plate 22, 22a, with itsphotosensitive target 13, are removed from the evaporation chamber andeach is sealed to the open end of two different envelopes 12. A methodof and means for forming the seal and other tube processing techniques,after the target 18 has been deposited, is described in a copendingapplication of B. H. Vine, Serial No. 648,094, filed March 25, 1957, nowUS. Patent No. 2,984,769, and assigned to the assignee of thisinvention.

The use of an open type evaporator boat is undesirable because theevaporants tend to spatter and cause spots on the photosurface. Whenopen boats were used, spot scrap may be as great as 99%. This scrap wasreduced to less than 20% by employing an enclosed evaporator whichoffered no direct line of sight between the evaporant source and target.FIGURE 5 illustrates a type of evaporator boat which has proved veryuseful. In an evaporation of two different materials, two suchevaporators are used, one for each evaporant.

The evaporator boat is directly heated with the ends of the boat 30clamped to electrical leads (not shown) and current (generally A.C.)passed through the boat. Two evaporant reservoirs are used for symmetryand should be filled with equal amounts of the evaporant. The evaporatoris composed of a bottom portion containing reservoirs 38 and vaporchannel 39 and a flat lid 37 in the center of which is punched a 0.10"vapor orifice 40. The lid is crimped over the edge of the bottom portionto make the unit vapor tight except for orifice 49. The orifice 40 islarge enough to prevent excessive pressure build-up within theevaporator, yet small enough to form approximately a point source forthe monitoring system. The evaporator may be formed from .022 tantalumsheet.

The monitoring techniques described above provide a knowledge of theamount and composition gradient of a photosurface as it is beingdeposited. In order to obtain a pre-specified composition gradient,however, it is necessary to control the relative ratio of evaporation ofthe component evaporands, using the monitored information to detectdeviations from the desired gradient. For satisfactory control severalfunctions must be performed, namely (1) monitoring, (2) computing, (3)controlling, and (4) recording. The monitoring function was describedpreviously and may be performed visually or by the use of knownelectro-optical techniques. Electromechanical equipment (not shown) maybe used to perform the functions of controlling and recording as well ascertain rudimentary compositions. The evaporation rate, from a boat 30,is a function of the current through the evaporator boat 30. Thiscurrent may be accurately controlled by means of known variableresistance devices (not shown) in series with the different evaporatorsand the resistance devices can be controlled so as to either increase ordecrease the evaporator current.

In order to provide a predetermined evaporator current schedule, towhich minor corrections can be made during the course of theevaporation, a tape (not shown) may be pre-punched with current increaseand decrease holes, in accordance with a preferred schedule, and runthrough an electrical sensing device at a constant rate. Electricalrelays may be used to operate the resistance devices as the currentincrease or decrease holes pass through the sensing device. Also,corrections in the preferred schedule may be supplied by the monitor,and applied to the tape, so that if trends in the corrections are noted,in several evaporations, the pre-punched schedules can be modified toreduce the number of corrections necessary during a subsequentevaporation. The observed interference bands may also be recorded on atape as they occur.

The monitor, using the interference bands and knowing the preferredschedule, can apply corrections to the preferred schedules to bring theevaporation into line. The monitor then, by his selection of the amountof correction, is serving as a computer. This function can be performedelectronically or electro-mechanically with relatively simple circuitryusing only the signal output of the interference band differentials.

The equipment, whose operation is briefly described above, permitssatisfactory controlled evaporations to be made by a single operator. Byemploying electro-optical monitoring, the operation can be madecompletely automatic. The evaporation could be processed unattendedwhile a complete record of each evaporation was being made.

One of the distinct advantages of some of the composition gradedphotosurfaces is the high sensitivity of such surfaces. The antimonytri-sulfide, antimony oXy-sulfide photosurface, for example, can be madeappreciably more sensitive than photosurfaces prepared from either ofthe components alone.

Thus, the invention provides a novel method of and means for monitoringthe amount and kind of deposit made from a plurality of sources. Sincevariations may be applied to a preferred schedule, changes that occur inthe system, such as a change in vacuum pressure during the evaporation,can be compensated so that the preferred layer may be deposited.

What is claimed is:

l. The method of depositing a photosensitive surface layer on a support,comprising the steps of simultaneously evaporating on said support aphotosensitive material from a first evaporator with said firstevaporator in a first position and evaporating photosensitive materialfrom a second evaporator with said second evaporator in a secondposition, positioning a member for receiving material from both of saidevaporators, positioning an apertured member for receiving material fromboth of said evaporators, positioning a monitoring plate on the oppositeside of said apertured member from said evaporators for receivingphotosensitive material from both said first and second evaporator andpassing through the aperture in said apertured member so that saidmaterial will land at spaced apart areas on said monitoring surface, andmonitoring the amount of photosensitive material deposited on saidspaced apart areas.

2. The method of controlling the deposit of photosensitive material on asurface comprising depositing photosensitive material from a firstsource onto said surface and through an aperture onto a first area of amonitoring member, depositing photosensitive material from a secondsource onto said surface and through said aperture onto a second area ofsaid monitoring member, said first and said second areas being spacedapart on said monitoring member, and monitoring the amount of materialdeposited on said monitoring member.

3. The method of making a photoconductive pickup tube comprisingdepositing a transparent conductive coating onto a transparent faceplate, evaporating at least one photoconductive material from at leasttwo evaporating boats spaced apart in a given plane and onto saidtransparent coating, positioning a monitoring means in spaced relationto said plane to receive said photoconductive material from both of saidevaporating boats, positioning a member having an aperture thereinbetween said monitoring means and said evaporating boats, whereby saidphotoconductive material passes through said aperture and is depositedon said monitoring means at spaced apart areas, positioning an electrongun in an envelope, and sealing said face plate with said transparentconductive coating and With the evaporated photoconductive material tosaid envelope.

2,391,280 Teal Dec. 18, 1945 a Vine Jan. 31, 1956 Ruedy May 8, 1956Korner et al. Jan. 27, 1959 Knochel Jan. 27, 1959 Marschka et a1. Apr.7, 1959 Auphan Sept. 29, 1959 Gunther May 31, 1960 FOREIGN PATENTS ItalySept. 27, 1954 Germany May 21, 1959 UNITED OFFICE CERTIFICATE 05FCORRECTION Patent No. 3, 127 226 March 31 1964 Charles Rector It ishereby oertifiedfithat ernorgpgeersin the above numbered patentrequiring correction and that theqeaid Letters Patent should read ascorrected below Column 3, line 44, for- "j'ib" read jig column 5 line 63for "compositions? read computations column 6, line 12 after "can""'insert -{t"hen -v-.

Signed and sealed this 17th day of November 1964.,

(SEAL) Attest:

ERNEST W. SWIDER I EDWARD J. BRENNER Anesting Officer I v I vCommissioner of Patents

1. THE METHOD OF DEPOSITING A PHOTOSENSITIVE SURFACE LAYER ON A SUPPORT,COMPRISING THE STEPS OF SIMULTANEOUSLY EVAPORATING ON SAID SUPPORT APHOTOSENSITIVE MATERIAL FROM A FIRST EVAPORATOR WITH SAID FIRSTEVAPORATOR IN A FIRST POSITION AND EVAPORATING PHOTOSENSITIVE MATERIALFROM A SECOND EVAPORATOR WITH SAID SECOND EVAPORATOR IN A SECONDPOSITION, POSITIONING A MEMBER FOR RECEIVING MATERIAL FROM BOTH OF SAIDEVAPORATORS, POSITIONING AN APERTURED MEMBER FOR RECEIVING MATERIAL FROMBOTH OF SAID EVAPORATORS, POSITIONING A MONITORING PLATE ON THE OPPOSITESIDE OF SAID APERTURED MEMBER FROM SAID EVAPORATORS FOR RECEIVINGPHOTOSENSITIVE MATERIAL FROM BOTH SAID FIRST AND SECOND EVAPORATOR ANDPASSING THROUGH THE APERTURE IN SAID APERTURED MEMBER SO THAT SAIDMATERIAL WILL LAND AT SPACED APART AREAS ON SAID MONITORING SURFACE, ANDMONITORING THE AMOUNT OF PHOTOSENSITIVE MATERIAL DEPOSITED ON SAIDSPACED APART AREAS.